Poly(isobutylene-co-isoprene), or IIR, is a synthetic elastomer commonly known as butyl rubber which has been prepared since the 1940's through the random cationic copolymerization of isobutylene with small amounts of isoprene (1-5 mole %). As a result of its molecular structure, IIR possesses superior air impermeability, a high loss modulus, oxidative stability and extended fatigue resistance.
Butyl rubber is understood to be a copolymer of an isoolefin and one or more, preferably conjugated, multiolefins as comonomers. Commercial butyl comprises a major portion of isoolefin and a minor amount, usually not more than 2.5 mol %, of a conjugated multiolefin. Butyl rubber or butyl polymer is generally prepared in a slurry process using methyl chloride as a diluent and a Friedel-Crafts catalyst as part of the polymerization initiator. This process is further described in U.S. Pat. No. 2,356,128 and Ullmanns Encyclopedia of Industrial Chemistry, volume A 23, 1993, pages 288-295.
Halogenation of butyl rubber produces reactive allylic halide functionality within the elastomer. Conventional butyl rubber halogenation processes are described in, for example, Ullmann's Encyclopedia of Industrial Chemistry (Fifth, Completely Revised Edition, Volume A231 Editors Elvers, et al.) and/or “Rubber Technology” (Third Edition) by Maurice Morton, Chapter 10 (Van Nostrand Reinhold Company © 1987), particularly pp. 297-300.
U.S. Pat. No. 5,886,106 issued Mar. 23, 1999 describes a halogenated butyl rubber with a halogen content of 0.5-2.5% and a content of non-halogenated double bonds of greater than 0.7 mol %, but preferentially 0.7-1.0 mol % with an anti-agglomeration control agent (i.e. calcium stearate (CaSt2)) in an amount of 1.25-2.2 wt %.
U.S. Pat. No. 7,446,151 issued Nov. 4, 2008 describes a process to produce high isoprene (IP) butyl rubber, specifically a polymer having a Mooney viscosity of at least 30 MU and a gel content of less than 15 wt % comprising repeating units derived from at least one isoolefin monomer, more than 4.1 mol % of repeating units derived from at least one multiolefin monomer and optionally further copolymerizable monomers.
In the production of butyl rubber, C13 (1-isopropenyl-2,2,4,4-tetramethylcyclohexane, C13H24) and C21 (1,1,5,5-tetramethyl-2-(1-methylethenyl)-3-(2,2,4-trim ethylpentyl)-cyclohexane, C21H40) cyclic oligomers having the structures C13 and C21 depicted below are produced. These cyclic oligomers are unsaturated, and halogenation of the butyl rubber also results in halogenation of the cyclic oligomers.

Such cyclic oligomers, especially halogenated cyclic oligomers, are undesirable in certain applications, for example in pharmaceutical seals, closures, blood collection stoppers, medical devices and food grade applications, due to the potential for the halogenated cyclic oligomers to migrate into/onto the pharmaceutical or food product, and then interact with the product and/or be introduced into a patient. Therefore a reduction in halogenated cyclic oligomer levels in halogenated butyl rubber is desirable.